1. Field of the Invention
The present invention relates to high performance multistage synchronization of pulsed radiation sources.
2. Description of the Prior Art
The ability to synchronize a passively mode locked laser to a reference, or to another laser, has many applications. Conventional methods of synchronizing two mode-locked lasers has accomplished timing jitters of, at best, a few hundred femtoseconds. Since it is now possible to generate pulses with a duration of less than 20 femtoseconds, improved synchronization is desirable, in order to take full advantage of the available time resolution.
Potential applications of improved synchronization of pulsed radiation sources include the generation of hard, bright x-ray beams via inverse Compton scattering, all electronic pump probe scanning, arbitrary repetition rate sum-frequency and difference-frequency generations, and extension of optical frequency comb bandwidth, as well as novel pulse generation and shaping techniques.
FIG. 1 shows two conventional arrangements for the mutual stabilization of two fs or ps mode locked lasers. In the left hand diagram, both lasers are independently locked to a reference oscillator. In the right hand diagram, laser 1 is locked to a reference oscillator and laser 2 is then locked to laser 1. Conventional schemes such as those shown in FIG. 1 typically achieve jitter of, at best, a few hundred femtoseconds.
While many other synchronization techniques for mode locked lasers and the like exist, they all suffer from the same disadvantages. The synchronization cannot be made accurate or efficient enough.
A need remains in the art for more accurate and efficient apparatus and methods for synchronizing mode-locked lasers and the like.
An object of the present invention is to accurately and efficiently synchronize mode-locked lasers and the like.
This object is accomplished by applying two or more stages of synchronization between the two pulse sources, each stage operating at a higher frequency than the stage before it. In this way the phase difference between the two sources can be quickly and easily acquired (and reacquired after interruptions) with the lower frequency stage, with control then passed to the higher frequency stage(s), which can take over once the two pulse trains are sufficiently synchronized by the first stage.
While a time domain description of the laser output shows an unending series of temporally narrow pulses, these lasers actually can operate under rather stable conditions, with successive pulse delay times and peak powers very much equivalent to the ones just preceding. In this case, a frequency domain description is also valid and quite useful. For example the summation of a long temporal series of equivalent pulses corresponds in the frequency domain to a xe2x80x9cpicket fencexe2x80x9d or xe2x80x9coptical combxe2x80x9d of frequencies, spaced rigorously by the inverse of the stable interpulse time interval. Each of the two fs lasers emits such a pulse train, and can be equivalently viewed as generators of optical frequency combs. Because of the strict mathematical relationship between the time and frequency domain descriptions, we can sense and effect temporal control by measuring frequency domain properties. Specifically, if the two lasers are to display the same interpulse time interval, they necessarily will have optical frequency combs with the same frequency separations between the comb lines. To implement a control system, then, it is powerful to make frequency domain measurements of the two separate pulse trains with separate high speed photodetectors. Of course there will be a heterodyne output given by each comb line beating with its two nearest neighbors, yielding a beat frequency matching the repetition rate. Comb lines beating with more distant brothers will lead to higher harmonic frequencies of the basic fundamental repetition rate.
This lowest frequency is used for the rough tuning control loop, because it has the largest unambiguous temporal range. Multi-frequency outputs of optical detectors coupled to the two lasers are filtered by pass-filters for their output at the fundamental frequency, for example 100 MHz. One of these channels passes through an adjustable rf phase delay before joining its partner in a balanced mixer, which functions as a phase detector, giving an output voltage dependent upon the relative phase of the two rf signals. A suitable controller converts this error output into a correction signal which is sent to one of the lasers to effect a stable phase lock, and thus accomplish a temporal domain synchronization of the optical pulse trains emitted by the two independent lasers.
Apparatus for synchronizing the repetition rates of two pulse radiation sources comprises a device for controlling the repetition rate and hence the timing of a designated one of the sources with respect to the other source in response to a control signal, a first synchronization stage for roughly synchronizing the two sources, by generating a rough feedback signal comprising the control signal, and a second synchronization stage for finely synchronizing the two sources, by generating a fine feedback signal comprising the control signal.
A repetition rate control device initially provides the rough feedback signal as the control signal to the designated source, and then gradually shifts to provide the fine feedback signal as the control signal to the designated source. For example, the sources may mode-locked lasers. In one embodiment, the lasers are Ti:Sapphire lasers, and the jitter between the two lasers is reduced to less than 15 femtoseconds.
If the lasers have a common repetition rate, the first synchronization stage generally synchronizes the fundamental frequency of the common repetition rate and the second synchronization stage synchronizes a harmonic of the common repetition rate. If the lasers have different repetition rates, the first synchronization stage synchronizes lower harmonics of the repetition rates and the second synchronization stage synchronizes higher harmonics of the repetition rates.
A third synchronization stage may be provided for extra-finely synchronizing the two sources, by generating an extra-fine feedback signal comprising the control signal. The extra-fine feedback signal might be based upon a combination of the laser beams within a nonlinear device. Or, the extra-fine feedback signal could be based upon a heterodyne beat of the laser beams.